The present invention pertains to aircraft windshear detection systems. It particularly refers to laser-based windshear detectors.
Some related art detectors have utilized the Doppler shift of light scattered by particulate matter (i.e., aerosols) dispersed in moving air. These Doppler sensors use either homodyne or heterodyne detection which requires a separate local oscillator and an interferometer. These sensors are sensitive to vibration and noise. The present invention does not require a heterodyne interferometer and separate local oscillator but a simple combination of a tunable laser and a filter. The invention makes use of backscatter from molecules (i.e., Rayleigh backscatter) and/or aerosol particles.
A technique using a transmission filter and a tunable laser has been disclosed in the related art. In this technique, the so called edge technique, a laser is placed at the edge of a transmission filter. A portion of the backscattered light goes through the edge filter while another portion together with a reference signal falls on detector which is an energy monitor. The Doppler shift is determined from a differential measurement of the frequency of the outgoing laser pulse and the laser return backscattered from the atmosphere. This is accomplished by measuring the normalized edge filter output for both the outgoing and backscattered laser signals on a per pulse basis and taking the difference in these outputs. Disadvantages of this approach are: all the power must be generated in one laser that must have good tunability and resetability features that generally cannot be obtained from one laser; and the laser must have good long-term stability; otherwise different portions of the slope of the filter lead to different scale factors on the read-out.